I'm so sorry, but it appears that you've found an obscure but harmful bug. Ink rats should not be able to activate pressure switches, but it seems this one is an exception. I have passed this information along to the developers and they will provide an update ASAP.
Unfortunately, now that you are in the room with the Bushling boss, you are effectively trapped and will have to start a new game. If you already have the leaf pencil, it's possible you could make your way over to the rain pencil if you have enough hearts and then use it to defeat the boss. But if you don't have the leaf pencil already, you are trapped indefinitely
I'm sorry for this, but thank you for bringing it to our attention so it does not happen to other players in the future!